1. Field of the Invention
The present invention relates to a Coriolis mass flow meter and, more specifically, relates to a small-sized mass flow meter preferably applicable to measurement of a minute flow rate.
2. Description of the Background Art
Since a Coriolis mass flow meter has an advantage of being capable of directly measuring a mass flow rate with accuracy, historically, its application started to measure a large flow rate, and has recently been expanded to measure a minute flow rate.
A principle of the Coriolis mass flow meter is as follows. When a central portion of a vibration tube through which a fluid is flowing in an axial-line direction is excited, Coriolis forces act in opposite directions in an inlet side portion and an outlet side portion of the vibration tube, and the Coriolis forces in the opposite directions cause generation of twist in the vibration tube. This twist is proportional to the mass flow rate. This twist of the vibration tube is detected as a vibration displacement, a velocity phase difference, or the like between the upstream side and the downstream side in a flowing direction of a fluid with the vibration exciter interposed therebetween, to obtain a mass flow rate from this phase difference or the like.
There have already been proposed vibration tubes having a variety of shapes in Coriolis mass flow meters. The vibration tubes can be broadly divided into a straight tube type which is straight and a curved tube type provided with a curved portion. The curved tube type is typically classified into a type of the inlet and the outlet of the vibration tube being located on the same side and a type of the inlet and the outlet being located on the opposite sides. As for the former type, namely the type of the inlet and the outlet being located on the same side, those having a U-shape in a planar view, a shape of a narrowed space between the inlet and the outlet, a loop shape, and the like are known. Further, the Coriolis mass flow meters are classified, based upon the number of vibration tubes, into a single tube type and a dual tube type provided with two vibration tubes.
A vibration tube using a metal material (typically, stainless steel) has been traditionally adopted to the Coriolis mass flow meter. Japanese Unexamined Utility Model Publication No. S64-15921 (Japanese Utility Model Application No. S62-107307) proposes a vibration tube formed by arranging a tube of a synthetic resin, such as a fluorine resin as a material having excellent corrosion proof properties, on the inner periphery of a metal tube in order to pave the way for application of the Coriolis mass flow meter to medical agents which are acid, alkaline and the like.
Further, Japanese Translation of PCT Publication No. H11-510608 discloses constituting a vibration tube of a plastic material having excellent corrosion proof properties, and cites as examples thereof polytetrafluoroethylene (PAFE), a tetrafluoroalkoxy polymer (PFA), and polyether ether ketone (PEEK).
Japanese Translation of PCT Publication No. H11-510608 above discloses a dual tube type Coriolis mass flow meter. In this Coriolis mass flow meter disclosed in Japanese Translation of PCT Publication No. H11-510608, it is disclosed that a combination of a coil for electromagnetic drive and a permanent magnet is adopted as a detector for detecting a vibrating state of each vibration tube, the coil being arranged on one vibration tube, the permanent magnet being arranged on the other vibration tube.
Further, Japanese Translation of PCT Publication No. H11-510608 proposes separately arranging wires, connected to coils of the vibration exciter and the detectors to the two vibration tubes and extending the wires outside. It goes without saying that the proposal of Japanese Translation of PCT Publication No. H11-510608 is made on the assumption that a controller (circuit board) for the detectors and the vibration exciter is arranged outside.
Japanese Unexamined Patent Publication No. 2003-207380 discloses a dual tube type Coriolis mass flow meter, to which two curved tube type vibration tubes that vibrate as a tuning fork are adopted, for the purpose of application to a fluid with a minute flow rate. It is pointed out as reasons for adopting the two curved tube type vibration tubes that a single tube type has a low drive efficiency and is unbalanced when vibrated, thereby causing the problem of occurrence of vibration leakage, and that the single tube type requires a frame structure with enhanced rigidity to support the detector, namely rigidity against vibrations. Further, Japanese Unexamined Patent Publication No. 2003-207380 proposes soldering a magnetic material such as silicon steel to the vibration tube, fixing a permanent magnet that magnetizes this magnetic material to a frame, and arranging a coil on this frame. Moreover, Japanese Unexamined Patent Publication No. 2003-207380 discloses coupling the extremities of the pair of vibration tubes with each other through insulating plates (brace bars), to form vibration nodes of the vibration tubes.
Incidentally, the vibration leakage is described below. Since a pair of vibration tubes ideally vibrates in a mirror-symmetrical manner, a vibration due to a self-excited vibration is offset on the frame. However, this is an ideal theory, and the tubes are in effect not completely mirror-symmetrical because of their material properties, shapes, assemblies, or the like being nonhomogeneous, nonuniform, or asymmetrical. Hence, the vibrating states in minute vibrations of the vibration tubes associated with the frame or an external pipe vary depending upon the assembled states thereof. This leads to an offset of a zero point of a measured value. This phenomenon is referred to as the vibration leakage. This vibration leakage occurs independently from a vibration from the outside as a disturbance element.
U.S. Pat. No. 4,756,198 discloses a looped dual tube type Coriolis mass flow meter that accepts a fluid from one side and discharges the fluid from the other side. In this Coriolis mass flow meter of U.S. Pat. No. 4,756,198, a vibration exciter and detectors are each configured by a combination of a permanent magnet and a coil, the permanent magnet being installed on the vibration tube, the coil being installed on the frame. This frame disclosed in U.S. Pat. No. 4,756,198 as a member to install the coil on is arranged between the pair of vibration tubes.